G. Scheitrum

High gradient RF breakdown study

Summary form only given. Stanford Linear Accelerator Center and UC Davis have been investigating high gradient RF breakdown and its effects on pulse shortening in high energy microwave devices. RF breakdown is a critical issue in the development of high power microwave sources and next generation linear accelerators since it limits the output power of microwave sources and the accelerating gradient of linacs. The motivation of this research is to find methods to increase the breakdown threshold level in X-band structures by reducing dark current. Emphasis is focused on improved materials, surface finish, and cleanliness. The test platform for this research is a travelling wave resonant ring. A 30 MW klystron is employed to provide up to 300 MW of travelling wave power in the ring to trigger breakdown in the cavity. Five TM/sub 01/ cavities have previously been tested, each with a different combination of surface polish and/or coating. The onset of breakdown was extended up to 250 MV/m with a TiN surface finish, as compared to 210 MV/m for uncoated OFE copper. Although the TiN coating was helpful in depressing the field emission, the lowest dark current was obtained with a 1 /spl mu/inch surface finish, single-point- diamond-turned cavity.

Design, fabrication and test of the klystrino

Summary form only given. The klystrino program was initiated to develop a high power RF source at W-band for tabletop accelerator, radar and communications applications. The goal of the program was to produce a 95 GHz, PPM focused, 100 kW peak power, 1% duty klystron. At W-band, both the small cavity dimensions and the required surface finish present fabrication challenges. In order to overcome these difficulties, the RF circuit for the klystrino is fabricated using an X-ray lithographic process called LIGA. The LIGA process can produce deep features (/spl les/ 3 mm) with submicron accuracy and excellent surface finish(< 50 nm). The RF design of the klystrino is straightforward with five regular cavities plus a five-gap extended interaction output cavity. The planar cross-section of the LIGA fabricated cavities requires 3-D simulation codes to model the non-axisymmetric fields. The rectangular magnets and butterfly-shaped polepieces also require a 3-D magnetostatic code to model the magnetic fields. Development of accurate 3-D simulations has been an ongoing issue in the klystrino program. The 110 kV beam voltage was chosen to make PPM focusing easier by reducing the plasma frequency. The length of the output cavity is proportional to beam velocity, so the high voltage also results in additional surface area to dissipate heat. Post-LIGA machining operations have presented the most difficult challenges to the klystrino development. Normal machining processes are pushed to the limit to achieve the one to two micron tolerances required for alignment and machining of the beam tunnel and coupling irises. Initial klystrino testing will start in February and RF performance data will be presented.

W-band sheet beam klystron design

Sheet beam devices provide important advantages for very high power, narrow bandwidth RF sources like accelerator klystrons. Reduced current density and increased surface area result in increased power capability, reduced magnetic fields for focusing and reduced cathode loading. These advantages are offset by increased complexity, beam formation and transport issues and potential for mode competition in the overmoded cavities and drift tube. This paper describes the design issues encountered in developing a 100 kW peak and 2 kW average power sheet beam klystron at W-band including beam formation, beam transport, circuit design, circuit fabrication and mode competition.

Pulsed rf breakdown studies

A series of experiments have been conducted to investigate the critical mechanisms involved in pulsed rf breakdown. This research has examined fundamental issues such as microparticle contamination, grain boundaries, residual gas, pulse duration, field emission, and the spatial distribution of plasma during a breakdown event. The motivation of this research is to gain a clearer understanding of the processes involved in breakdown and to determine methods to increase the breakdown threshold thereby increasing the available power in high power microwave sources and accelerator components.

Plasma deposition of oxide cathodes

Vacuum arc deposition is employed to create a barium and/or strontium plasma which is subsequently deposited/implanted onto a nickel cathode substrate. The primary motivation for this work is the critical need for a reliable, repeatable, long-lived thermionic cathode for the production of high power, microsecond duration microwave pulses; such cathodes may also have applicability for lower current density continuous wave devices. This novel approach to manufacturing an oxide cathode eliminates the binders that may subsequently (and unpredictably) poison cathode emission. Removal of the poisoning mechanisms has yielded oxide cathodes capable of emission densities in the 20 A/cm2 regime. Cathode lifetime and emission may be varied via the control over the deposition parameters such as coating thickness, implantation energy, and plasma stoichiometry. The deposition is performed by generating a cathodic arc discharge at the surface of a barium or barium-strontium alloy rod. The metal plasma thus created is th...

High power W-band klystrons

The development of W-band klystrons is discussed. Modeling of the klystron performance predicts 100 kW output power from a single klystron. The permanent magnet focusing and small size of the circuit permit combination of multiple klystrons in a module. A six-klystron module in a single vacuum envelope is expected to produce 500 kW peak power and up to 5 kW average power. The critical issues in the W-band klystron development are the electron beam transport and the fabrication of the klystron circuit. Two microfabrication techniques, EDM and LIGA, are being evaluated to produce the W-band circuit.

3-D simulations of multiple beam klystrons

The MAGIC3D simulation code is being used to assess the multi-dimensional physics issues relating to the design and operation of multiple beam klystrons. Investigations, to date, include a detailed study of the mode structure of the cavities in the 19-beam hexagonally packed geometry and a study of the velocity spread caused by the cavity mode’s field profile. Some attempts to minimize this effect are investigated. Additional simulations have provided quantification of the beam loading Q in a dual input cavity, and optimization of a dual output cavity. An important goal of the simulations is an accurate picture of beam transport along the length of the MBK. We have quantified the magnitude and spatial variation of the beam-line space charge interactions within a cavity gap. Present simulations have demonstrated the transport of the beam through three cavities (the present limits of our simulation size) without difficulty; additional length simulations are expected. We have also examined unbalanced beam-line scenarios, e.g., one beam-line suppressed, and find little disturbance to the transport in individual cavity tests, with results for multiple cavity transport expected.

RF Breakdown Studies in Tungsten and Copper Structures

This paper reports on experimental results from the SLAC NLC accelerator structure closeout program, and discusses a study that was conducted to improve the smoothness of machined tungsten for use in high gradient structures. At the Next Linear Collider Test Accelerator (NLCTA), an X‐band (11.424 GHz) structure was operated at a lower temperature to determine whether this would decrease the low rate of breakdowns that still occur after initial processing. Also, various vacuum venting experiments were performed to determine the impact of air, airborne particulates, and oxidation on the performance of a processed accelerator structure. As part of a more long‐range high‐gradient structure development program, alternative materials to copper are being explored. The CLIC study group at CERN has conducted several accelerator experiments at 30 GHz with structures that have tungsten and molybdenum iris inserts. SLAC has also tested versions of the CLIC 30 GHz design scaled to 11.424 GHz. The results have prompted...

High power microwave research at SLAC

Summary form only given. This presentation will report on the efforts at SLAC to produce high power microwave sources for the Next Linear Collider (NLC). The NLC as outlined by SLAC will require 4000 X-band klystrons operating at >50 MW with a 1.2 /spl mu/s pulsewidth. SLAC has recently developed and tested an X-band klystron up to 75 MW. Design and performance data for this tube will be presented. Since the current X-band klystron uses a 25 kW solenoid to focus the electron beam, the power budget for 4000 of these solenoids would be 100 Mw. This was deemed excessive and both superconducting solenoids and PPM focusing were investigated. SLAC has designed and tested a 460 kV, 188 A, PPM focused, beam tester to evaluate PPM focusing for the X-band klystron. Initial results have produced better than 99.9% transmission for the samarium-cobalt, PPM focused device. Both the beam tester results and the design of a PPM focused klystron will be presented.

Micro/Nanotechnology and Terahertz Radiation Discussion Session

Summary of the background presentation and synopsis of the subsequent discussion on microfabrication and THz sources.